DE 10 2013 005 624 A1 discloses a hollow cylindrical ring filter element in a filter device that is employed for separation of oil droplets from an oil-containing air stream originating from the crankcase of an internal combustion engine. The ring filter element is flowed through radially from the exterior to the interior wherein, adjacent to the externally positioned inflow side of the filter element, a pressure-relief valve is arranged that opens automatically for discharge and bypassing the filter element when the pressure of the air stream at the inflow side surpasses a threshold value. When the pressure-relief valve is open, the unpurified air stream is discharged into the environment.